A major limiting factor in ULSI interconnect technology is RC time delay introduced by the coupling of metal-insulator characteristics. An efficient interconnection scheme for advanced ULSI circuits requires materials with low effective time constants. In this regard, metals with low resistivity such as copper and the noble metals are emerging as materials of choice.
Some of the issues to be addressed in order for Cu-based interconnects to be a viable choice, especially as integration density continues to rise, involves the processes for patterning the copper lines, the prevention of diffusion of the copper into the underlying active substrate, and the prevention of air corrosion on the surface of the copper. The latter mentioned issues being the most important at this time.
Currently, new barrier materials deposited between the copper and the substrate or dielectric layer and processes related to their application to reliably prevent or substantially reduce copper contamination of the underlying material are under investigation. One reported barrier layer is a plasma assisted silicon nitride deposited film.
In order to passivate the copper surface subsequent to formation of the patterned copper interconnect lines, to both avoid interaction with adjacent dielectrics, e.g. SiO.sub.2, and to avoid air oxidation, the current state of the art employs various techniques including the formation of a silicide coating over the exposed copper; encapsulation with thin deposited inorganic films such as a-Si or Si.sub.3 N.sub.4 ; alloying of the copper surface to form Cu.sub.3 Ti, Cu.sub.3 Pd or CuAl.sub.2 ; and implantation of boron into the copper. These passivation steps usually involve harsh conditions such as elevated process temperatures (300.degree. C.-400.degree. C.), ion implantation and/or exposure to a plasma, which could adversely effect device fabrication. Also, these processes are relatively costly and time consuming. It would therefore be advantageous to find a less costly and/or less time consuming passivation process which preferably can be done in a single step without the need for specialized expensive capital equipment, and which does not employ harsh processing parameters. It is also desirable to find alternative barrier materials and processes to prevent migration of copper into the underlying substrate.
In the unrelated field of electrochemistry wherein gold electrodes are immersed in liquid electrochemical plating baths, spontaneously self-assembling mono-layer organic films have been employed to modify the surfaces of the gold electrodes. Such films have also been employed as biologically active membranes. These films have been described in various publications such as R. J. P. Williams, Self-Assembling Surfaces, Nature 332, 393, Mar. 31, 1988; I. Rubenstein et al., Ionic Recognition and Selective Response in Self Assembling Monolayer Membranes on Electrodes, Nature 332, 426-429, 1988; S. Steinberg et al., Ion-Selective Monolayer Based on Self Assembling Tetradentate Ligand Monolayers on Gold Electrodes. 2. Effect of Applied Potential on Ion Binding, J. American Chemical Society 113, 5176-5182, 1991; S. Steinberg et al., Ion-Selective Monolayer Based on Self Assembling Tetradentate Ligand Monolayers on Gold Electrodes. 3. Langmuir 8, 1183-1187, 1992; and M. M. Walczak et al., Modified Electrode Surfaces, The Electrochemical Society, 4th Edition, 39-40, 1997, all of which are incorporated herein by reference. We have now discovered that these same kinds of films can be employed to provide a passivation layer to prevent air corrosion of copper and more particularly to passivate copper interconnects in ULSI integrated circuits to substantially eliminate corrosion of the interconnects without any detrimental effects to the IC device. The same layers can also provide an effective barrier layer between the copper and its underlying substrate.